Electric motors are used to convert electric energy into mechanical energy. In electric motors of normal construction, it is possible to distinguish certain basic components, such as a rotor fitted to be rotatable, a rotor axle, a stationary stator, bearings and end shields. The rotor is supported by the bearings. Generally a small air gap is left between rotor and stator.
The operation of rotating multi-phase alternating current machines, such as multi-phase synchronous and asynchronous motors, is based on a magnetic field rotating within the machine. A multi-phase stator winding is formed in such a way that, when a sinusoidal voltage is fed into the phase windings—the voltages fed into the phases having a phase shift of 360/n degrees between them, where n is the number of phases—the currents flowing in the stator windings produce in the air gap of the machine a circulating magnetic field, whose interaction with the magnetic field of the rotor winding causes the rotor to rotate. In synchronous motors, the magnetic field of the rotor winding is typically generated either by permanent magnets or by a direct current fed into a rotor excitation winding. In asynchronous motors, the excitation of the rotor winding is generally effected via the voltages and currents induced in the rotor winding by the magnetic flux generated by the stator current.
The magnetic flux density distribution in the air gap is to be made as sinusoidal as possible. The rotary motion of the rotor is produced by the action of the sinusoidal fundamental wave of magnetic flux density, but in practice the magnetic field acting in the motor also contains harmonics, i.e. harmonic components of the fundamental wave.
The harmonics of the magnetic flux density generate extra force components between stator and rotor. Moreover, the magnitude of the torque fluctuates (torque ripple) and additional losses occur in the motor.
Harmonic components are created in the air gap flux density as a result of both discontinuity of the windings on the stator and rotor circumferences and variation of permeance in the air gap. The stator winding is generally concentrated in slots and coil groups, so that the magnetomotoric force produced in the air gap by the winding is not sinusoidally distributed. Variation of permeance in the air gap is caused e.g. by possible stator and rotor grooving, open poles and magnetic saturation. The harmonics of the magnetic field of an electric motor can be divided into harmonics caused by the rotor and those caused by the stator.
The windings of electric motors are traditionally distributed windings, wherein the coils of different phases are fitted in an interleaved fashion such that the area delimited by each coil also contains coil sides of other phases. Specification U.S. Pat. No. 6,581,270 describes a method for manufacturing a stator in which the coil sides are distributed into several slots in the pole area of the motor. As the coil sides of the same phase are located at a large distance from each other in the pole area, the end windings are long. A significant proportion of the conductor material used in the motor windings is not exploited, because the end windings generate no torque but cause losses and require space. Further, because in this construction the end windings of different phases cross with each other, the risk of short-circuit between coils is increased. The end windings thus require additional insulation. The winding work for preparing such windings also comprises several operations and often has to be carried out manually.
A motor provided with traditional distributed windings is also bulky and heavy due to the long end windings and the fact that the coils of a winding are usually distributed in many slots in the area of a pole pair. This is a disadvantage especially in motors used in elevators, because elevators are increasingly being implemented using machine solutions in which the machine is placed between a guide rail and a wall of the elevator shaft. Therefore, a large size and weight of the motor is a drawback.
During recent years, investigations have been undertaken to explore concentrated fractional-slot windings, because these provide solutions to certain problems associated with traditional windings. In a concentrated winding, the coil sides of the same coil are placed in adjacent slots. Thus, the end windings are shorter and do not take up so much space as in traditional windings.
A problem with concentrated windings is that, as the number of slots reserved for the winding per motor pole is smaller than in traditional motors, the magnetomotoric force produced in the air gap by the winding deviates greatly from a continuous sinusoidal pattern, thus containing more harmonics than in traditional windings. These harmonics produce both torque ripple and eddy currents in the motor.
Specification U.S. Pat. No. 6,894,413 discloses a generator in which the rotor is magnetized by permanent magnets and the stator has a concentrated fractional-slot winding. In the specification, the rotor diameter is defined by the equation:D≧0.00045×Pout where D stands for the rotor diameter and Pout for the power produced by the generator. Thus, for example, the rotor of a 5 kW generator has a diameter of at least 2.25 m. This is therefore a sizeable multi-pole and slowly rotating generator, which may be applied e.g. as a wind power generator. In the specification, harmonic components of the generator output voltage are also determined for certain different geometry combinations. The specification discloses which combinations can be utilized to reduce these harmonics, because, according to the specification, they cause eddy currents and consequently power losses in the generator. This specification also discloses the idea that the rotor can be assembled from thin laminated steel plates to reduce eddy currents, and likewise the idea that the rotor may be made from massive iron but the iron part is divided into segments to minimize eddy current losses.
In an effort to eliminate the torque ripple caused by a concentrated winding, e.g. a small slot opening has been used. Such a solution is disclosed at least in specification U.S. Pat. No. 6,882,080. According to this specification, a small slot opening reduces torque ripple, but the solution has the drawback that it is difficult to make the windings on a finished motor frame. Specification JP3451263 proposes a solution where the phase conductors are wound around the poles before assembly of the motor. This involves new stages of work in the assembly of the motor, which retards the production of the motor and increases the manufacturing costs.